Method of compacting made-up ground and natural soil of mediocre quality

ABSTRACT

A method of consolidating damp foundation soil, e.g., natural or synthetic clay or silt alone or mixed with sand, includes a number of cycles each including a dynamic force phase in which dynamic superimposed loads of at least 500 to 10,000 tons are applied to the soil to fluidize it, and a rest phase, possibly several days, during which the interstitial water escapes and the soil is restructured. Optimum parameters are ascertained by the use of a model testing rig comprising an expansible chamber to contain a sample of the soil, and means for applying static and dynamic pressure to the contents and measuring the relevant parameters.

United States Patent Menard Aug. 12, 1975 [54] METHOD F COMPACTINGMADEUP 2,236,759 4 1941 Lyman 61/35 GROUND AND NATURAL SOIL 0F 2,897,9078/1959 Blount et a]. 404/133 X 3,067,657 12/l962 Wiekhorst 404 133MEDIOCRE QUALITY 3,478,656 11/1969 McDonald 61/36 R x Louis Menard, 54Ave. de la Motte-Picquet, Paris, France Filed: May 21, 1973 Appl. No.:362,147

Related U.S. Application Data Continuation-impart of Ser. No. 183,397,Sept. 24, 1971, abandoned.

Inventor:

References Cited UNITED STATES PATENTS 6/1908 Kellner 404/133 UX ll/l927Friz 61/36 R Primary Examiner-Philip C. Kannan Attorney, Agent, orFirm-Alan H. Levine ABSTRACT A method of consolidating damp foundationsoil, e.g., natural or synthetic clay or silt alone or mixed with sand,includes a number of cycles each including a dynamic force phase inwhich dynamic superimposed loads of at least 500 to 10,000 tons areapplied to the soil to fluidize it, and a rest phase, possibly severaldays, during which the interstitial water escapes and the soil isrestructured. Optimum parameters are ascertained by the use of a modeltesting rig comprising an expansible chamber to contain a sample of thesoil, and means for applying static and dynamic pressure to the contentsand measuring the relevant parameters.

7 Claims, 4 Drawing Figures FATENTEBAUB 1 2197s 3, 898 844 SHEET 1 TM/MZ10- PATENTED AUG 1 2 I975 SHEET 2 E pl po 0 50 100 150 5 1O 15 20PATENTEU AUG 1 21975 3, 9 44 SHEET 3 FIG. 3

PAH-INTEL] AUG] 21% 3, 898.844 SHEET 4 METHOD OF COMPACTING MADE-UPGROUND AND NATURAL SOIL OF MEDIOCRE QUALITY This application is acontinuation-in-part of copending application Ser. No. 183,397, filedSept. 24, 1971, now abandoned.

The invention relates to the consolidation of soil to a great depth, sothat it can be used as foundation soil.

An object of the invention is to provide a method for very considerablyincreasing the density and bearing capacity of a foundation soil, themethod being particularly applicable to the consolidation of soil whichit is thought impossible to compact, e.g., clay or clay and sand orsilt, the consolidation being to a great depth, which is particularlyimportant for virgin soil and under-water embankments.

When the soil is soft, the foundations are generally constructed onpiles, allowing for super-imposed loads due to negative friction (theaction of the embankment on the pile shaft). These methods arerelatively expensive for relatively light structures (buildings lessthan five stories high) or widely distributed structures such asfactories. They are also difficult (e.g. the junction between thestructure on piles and any slabbing borne by the embankment).

In many cases, the embankment is made of materials having good intrinsicproperties (e.g., broken stones, sand, etc.) and their unfavorablebehavior (e.g., high compressibility and low bearing capacity) is duemainly to a lack of density and to the presence of voids.

The quality of an existing embankment can be improved in known manner bymoving static or vibrating compaction rollers or heavy excavatingmachinery over the embankment. Unfortunately, owing to the small weightof the roller to 50 tons), compaction is negligible beyond a depth of 50to 90 cm, and, in order to obtain appropriate results, the embankmenthas to be completely excavated and replaced in 30 to 90 cm layers, eachof which is compacted successively (a method used in road constructionor earth dams). This method is unsuitable for existing embankments orfor embankments in process of construction by dumping into water (e.g.,blocking up a sand-pit after use, or embankments for rivers and moreparticularly seas).

The invention is based on the efficient use of a complex phenomenon, andcomprises simultaneously fluidizing the soil in the mass and exertingvery considerable dynamic forces at its surface.

A soil is fluidized when its solid structure is so destroyed that thematerial behaves like a suspension of particles in a liquid phase(water). Hitherto, this phenomenon has resulted from chance naturalcauses (e.g., earthquakes) or artificial causes (e.g., buildingoperations) and has been considered completely undesirable, with theresult that the soil would be abandoned and considered unsuitable forany subsequent use as a foundation soil.

After systematic research and tests, it has now been discovered on thecontrary that fluidization, if brought about efficiently and incontrolled manner, may provide a very effective method of consolidatingsoil, more particularly clay, clay and sand, or silt and sand.

The method has two characteristic features, the first relating to theenergies used during the method and the second relating to the sequenceof the operations in the method.

According to the invention, fluidization is brought about by subjectinga great thickness of soil to considerable dynamic pressures adapted tobreak the solid structure thereof, i.e., usually pressures of 3 to 40bars, which can be achieved by dynamic forces reaching values of atleast 500 to 10,000 tons.

The method may comprise successive cycles, each cycle comprising aninitial phase corresponding to the application of a dynamic fluidizingpressure and a subsequent rest phase for removing pore water andenabling the material to be restructured, forming a new soil which isdenser and stronger than the original soil. The new soil can be eitherused as foundation soil after a single cycle or subjected to furthercycles.

The method can produce much more rapidly a settlement which is at leastequal to the settlement obtained over a long period under a staticsuperimposed load of several bars."

In a typical embodiment of the method according to the invention, thesurface of the soil to be compacted is subjected. to dynamic pressuresof the order of 3 to 40 bars, i.e., usually of several tens of bars, onan area of at least I to 10 m so as to produce dynamic stresses ofseveral bar's, e.g., at least 2 to 5 bars, at a great depth (say 3 to 10m) over large areas (say 10 to 40 m and a very large volume (say to1,000 m This corresponds to dynamic forces of 500 to 10,000 tonnes.

In each case, the skilled technician will determine the amount ofpressure and the areas involved, depending on the characteristics of thesoil before and during processing, and on the desired results.

To this end, a device may be used which will be described hereinafterand which also forms part of the invention.

The method, i.e., basically the production of dynamic fluidizationpressures, can be worked by any appropriate means, e.g., by droppingweights on to the soil or by the use of explosives.

1n the firstcase the weights may for example be from 6 to 50 tons andmay be dropped from 6 to 20 meters.

Under these conditions, the thickness of material which can be compactedin a single layer reaches 10 to 20 meters, which is incomparablysuperior to known methods, which are limited to a fraction of a metre orat most to 2 meters if the largest existing static or vibrating machinesare used.

In some cases the methods can be made more efficient (i.e., quicker andyielding a better result) if the top meter of soil is made ofhigh-quality material, i.e., draining material having a high angle ofinternal friction, before the first cycle or during the cycles. Theoriginal soil may be used or material may be added.

One specific embodiment of the method will now be described by way ofexample with reference to the accompanying drawings in which:

FIGS. 1 and 2 are graphs showing the changes in material subjected to acycle according to the method;

FIG. 3 is a graph showing changes in a soil subjected to a number ofcycles according to the method; and

FIG. 4 is a diagram of a device facilitating the process according tothe invention.

As indicated in FIG. 1 the method comprises one or more cycles which, oreach of which, includes a dynamic force phase and a rest phase. Duringthe dynamic force phase a number of heavy impacts are delivered to thesoil to be consolidated, either by means of explosives, or by dropping aweight of between 6 and 50 tons from a height of from 6 to 20 meters.FIG. 1 sh; ws eight such impacts, distributed over part of a day. Thisphase has the effect of closing the voids until the soil is saturatedwith water (after five impacts in the example shown) and therebyfluidized or'liquefied.

Thereafter the rest phase, which may last for several days or weeks(e.g., 2 weeks), provides an opportunity for the water to escape,allowing restructuring and consolidation of the soil.

During fluidization, the structure of the material is broken; all theintergranular stresses are taken up by the interstitial liquid, thepressure of which becomes equal to the weight of the soil above, whichis usually more than double the hydrostatic value.

Fluidization occurs simultaneously with an instantaneous settlement,which depends on the particle size of the soil and its initial voidratio, but which is rarely less than 3 percent of the total thickness ofthe compressible layer. The settlement is particularly spectacular inpeat and silty soil containing an appreciable percentage of organicmatter. The explanation is fairly simple: the soils always contain somegas (air, methane, etc.) and consequently behave like a solid-liquid-gascomplex or stack of hydropneumatic capacities. The dynamic stressreduces the volume of the gas phase, producing an instantaneous and veryappreciable total settlement.

The interstitial water, when suddenly subjected to a considerablepressure gradient, [has no difficulty in opening the structure formed bythe solid particles and previously fluidized by theshock waves,resulting in an internal breakdown and a multiple branching drainagenetwork.

As the interstitial pressure dissipates,-the network of intergranularstresses reforms and the material is 'restructured; the mechanicalstrength increases rapidly during the entire period of dissipation ofwater pressure, and then increases more slowly under the effect ofsubsequent thixotropic phenomena.

The energy used per m and the corresponding reduction in volume areshown by curves 1 and 2 respectively; the interstitial pressure, asa'percentage of the fluidization pressure, and the bearing} capacity(e.g., the limiting pressure measured by the pressure gauge) are shownby curves 3 and 4. Ascan be, seen, therefore, the reduction in volumeincreases substantially proportionally to the energy applied, w hereasthe interstitial pressure remains below the fluidization pressure, sothat the saturation energy canbe thus defined; subsequently, theinterstitial pressuredecreases rapidly with time, from the maximumreached during the application of dynamic pressure. The bearingcapacity, which is characterized by an initial value (Pl becomes verysmall during the fluidization phase, and then rapidly increases duringthe dissipation of the interstitial pressure and reaches a final value(P1) considerably above the initial value.

Energy may be applied in a single cycle in dry or very permeable soil.In other cases it is more usual to have anumber of cycles, the resultsof which are shown in FIG. 3. A second cycle does not begin until afterthe dissipation of the interstitial pressure produced during thepreceding cycle; so that, for example in the case of saturated siltysoil the process may take a total of several months.

Clearly, the required number of cycles increases as the material becomesless permeable, which limits the practical utility of the method in thecase of very impermeable clayey layers.

In order to obtain the best results, according to the invention thedimensions and shape of the tamping weight are selected in dependence onthe characteristics of the soil treated. The optimum efficiency is obtained when the tamper is driven in about to percent of its diameter orequivalent dimension at each blow.

' The tamper should also be slightly frustoconical so as to reducelateral friction and suction when it is withdrawn from the soil.

With regard to the plan distribution of energy, it may be advantageous,depending on the desired result, to finish tamping at a single placebefore moving to the next place, or, on the contrary, to tamp each pointon the elementary soil square (e.g., 4 X 4) at least once before thesecond or third series of tamping operations.

The method is also very advantageous in the case of underwater or drycoarse rocks (sea or river dykes or dams), sand and gravel, dry orunderwater coarse sand, moraine soil, or fine Fontainebleu sand (dry).

It is relatively advantageous in the case of silt con taining a smallproportion of water, loess and relatively dry sandy clay.

The method according to the invention can be facilitated by using adevice adapted to simulate the fluidization operation on a sample ofsoil and thus to determine the saturation energy (fluidizationthreshold), the time taken for the interstitial pressure to dissipate,and the increase in settlement in dependence on the energy applied. Suchan apparatus is shown in FIG. 4.

DESCRIPTION OF THE TESTING APPARATUS The apparatus comprises a closedchamber 1, e.g., a cylindrical tank of about liters capacity, havingdeformable side walls made for example of reinforced rubber or syntheticelastomer sothat lateral pressure can be measured during the differentoperations. The

chamber is closed at the top by a piston in the form of a plate 2 forapplying static or dynamic pressure to the contents of the chamber, anda disc 3 is disposed between plate 2 and chamber contents 4 in order todry the contents. The piston has a horizontal cross-sectioncorresponding to the cross-section of the sample of soil contained inthe chamber. I

The chamber forms an expansible jacket, in which the soil sample isdisposed in successive layers statically consolidated at a pressurewhich is a function of the resistance of the soil in situ. The contentof water introduced corresponds to the actual water content of thematerial. I

The disc 3, which is used for draining, is made, for example, of porousbronze or, better, of coarse sand.

If required, the disc can also form the piston 2, used for transmittingadequate static or dynamic pressures to the material in the chamber. Thepressures are supplied by any suitable means, e.g., by a jack 5 and atup or hammer 6 striking an anvil 7 secured to a rod 8 fixed to thepiston 2.

In the drawing, the jack acts on the piston 2 via a stirrup 9 which itpushes downwards, while the rod 8 extends through the stirrup 9.

The apparatus comprises measuring devices, including a detector 10 (orpressure gauge) for measuring the pressure exerted by the chambercontents on the deformable side wall, a detector 11 connected to thebottom end 12 of the chamber in order to measure the interstitialpressure, i.e., the degree of fluidization of the 1m to cm.

V0 I000 to 3000 cm Initial height of sample: Variation in height ofsample: Initial volume of sample: Static pressure maintained on head ofsample: P\',, 0.5 bar for standard test) Energy transmitted to sample inmeter-tones/m Lateral pressure exerted by sample on wall of mould atbase of sample: Ph Interstitial pressure at base of sample, minus theheight of water kept in the tank (since the test is performed undersaturation): P,-

III. Performance of Test A. Preparation of sample:

The sample is taken from a boring, taking care to avoid any change inthe water content. It is placed in the tank in lO-cm layers, each ofwhich is maintained at a pressure p equal to the limiting pressuremeasured in situ (with the pressure gauge).

A draining layer of coarse sand and the piston are positioned at the topof the sample. The following recommended static load should be keptconstant during the entire test:

I1 depth of middle of layer to be compacted,

I11 depth of water table,

y density of soil on saturation.

Usually, the average value:

p,. 0.5 bar is used in calculation. 40

It may be desirable to perform a' first lateral confinement test at thisstage of preparation, and to measure the consolidation modulus of thesoil before compaction. In all cases, the dynamic consolidation phasedoes not start until the interstitial pressure produced by the staticload has dissipated and been reduced to the lst Phase:

Total energy applied: Measurement of All Measurement of 1,, Measurementof 1, 2nd Phase:

Rest phase.

Ali, p,,, p, are measured until the interstitial pressure conditionshave returned substantially to their initial value, or at least until:

3rd and 4th Phases: Energy applied during phase: Total energy appliedsince the beginning of test:

13 I6 20 metre-tonnes/m Measurement of Ah, 12, p,-, etc., in dependenceon the known energy for the initial phase and the time during the restphase.

5th and 6th Phases:

Identical with the preceding. At the end of the test, a second lateralconfinement test is made, doubling the number of levels and using thesame pressure variations.

C. Use of results:

Determination of saturation energy:

This is the energy corresponding to the total fluidization of the soil.Any further application of energy produces very little settlement andthe increase of compaction is insignificant.

Determination of the settlement curve in dependence on the appliedenergy:

a. initial settlement curve for each phase:

Ah,- instantaneous variation in height of sample after application ofenergy during phases 1, 3, 5.

2A]: is plotted against the total energy applied to the sample (i.e., 3,3+3, or 3+3+3 meter tons/m b. total settlement curve:

This is the variation in the height of the sample, measured at the endof phase 2, 4 and 6 and plotted against the total energy applied. In thecase of a draining soil, the total settlement curve is identical withthe initial settlement curve.

D. Application to tamping work The laboratory results can be used todetermine the characteristics of the dynamic consolidation to beperformed in situ. The most important characteristics are:

the dissipation time,

the saturation energy,

the settlement.

The times Tto be allowed between each phase in situ should beproportional to the dissipation time t measured in the laboratory, atleast if one neglects the breakdown of the soil produced by theinterstitial pressure:

time between phases (in situ) dissipation time (in laboratory) H, halfthe thickness of the compressible layer to be compacted. The above valueis in fact an upper limit of T.

The energy for each phase should in no case be greater than thesaturation energy measured in the lab-- oratory.

The settlement AH of the soil is substantially proportional to thevariation in the volume of the sample for each compaction stage.

When a consolidation operation is being planned, the first value to beestimated is the minimum required settlement. This can be estimated as afunction of the measured resistance of the soil before tamping, and as afunction of the required resistance.

As a first approximation, each doubling of the initial resistance of thesoil should correspond to an increase d( a's) of 4 to 5 percent in thedensity ds of the same soil.

Thus, if the bearing capacity measured with the pressure gauge is pl 3bars, and if it is required to obtain lO-l2 bars at the end of theoperation, the density of the material must be increased by at least 8to 10 percent during the compaction operation:

When the increase in density Ali 11,,

has been determined, it is necessary to decide on the most economiccompaction process for obtaining the desired increase within anacceptable time. Preferably the energy chosen is the saturation energy,and the cor-' responding settlements produced in the sample are noted.

Ah 2 I!" Nil Ah n 11,,

The number n of phases is determined by the relation:

Ah H

Ah I

Ah A112 Ahn All The maximum time T between phases in situ isautomatically given by the dissipation time in the laboratory:

Consequently the total duration of operation is: T=Tl +T2+....Tn

sideration in the project, the number of phases or the time betweenphases can be reduced, though this substantially increases the energyrequired for compaction.

FIG. 4 shows a device wherein the chamber and the jack controllingstatic pressure are borne by a single support 14; it will be understood,however, that this is not essential.

By way of example, FIG. 2 of the accompanying drawings shows a pressureprofile or cross-section for a clayey silt soil. Curves are shown on theleft for the deformation modulus and on the right for limiting pressuresplotted against the depth in metres, shown as the ordinate.

The chain-line curves show values measured before compaction, thechain-dotted curves show results obtained after compaction after theinterstitial pressure has dissipated, and the continuous curves show theresults measured after 9 months. 1

In the claims, the word soil covers any kind of soil as stated above andspecially natural or synthetic, moist or saturated, clay, clay-sand,silt or sand soil.

What l claim as my invention and desire to secure by Letters Patent is:

1. A method of consolidating damp unfluidized soil, said methodcomprising the steps of: applying a dynamic load to the soil until thevoids in the soil are reduced to bring the soil to fluidization;discontinuing the application of the dynamic load after the soil hasbeen compacted to fluidization; and allowing the water to drain'from thesoil, during a rest phase.

2. A method according to claim 1, which includes determining the valueof the dynamic loads to be applied, the duration of the rest phase, andthe extent of settlement by simulating the fluidizing operation on asample of soil prior to compacting the soil in situ.

3. A method according to claim 1, which includes an operationintroducing dynamic stress cycles of at least 2 to 5 bars at a depth ofat least 5 to 20 in over an area of at least 10 to 40 m and a volume ofat least to l,OOO m.

4. A method according to claim 3, in which the said operation comprisesa plurality of cycles in each of which dynamic forces of at least 500 to10,000 tons are applied to the surface.

5. A method according to claim 1 in which the load is applied by atamper which is driven in about 10 to 20 percent of its equivalentdiameter at each impact.

V 6. A method according to claim 1, which includes at least twosuccessive cycles each comprising a dynamic pressure phase, followed bya rest phase.

7. A method according to claim 1, which includes an operation whereindraining materials are used to form the top part of the soil.

1. A method of consolidating damp unfluidized soil, said methodcomprising the steps of: applying a dynamic load to the soil until thevoids in the soil are reduced to bring the soil to fluidization;discontinuing the application of the dynamic load after the soil hasbeen compacted to fluidization; and allowing the water to drain from thesoil, during a rest phase.
 2. A method according to claim 1, whichincludes determining the value of the dynamic loads to be applied, theduration of the rest phase, and the extent of settlement by simulatingthe fluidizing operation on a sample of soil prior to compacting thesoil in situ.
 3. A method according to claim 1, which includes anoperation introducing dynamic stress cycles of at least 2 to 5 bars at adepth of at least 5 to 20 m over an area of at least 10 to 40 m2 and avolume of at least 100 to 1,000 m3.
 4. A method according to claim 3, inwhich the said operation comprises a plurality of cycles in each ofwhich dynamic forces of at least 500 to 10,000 tons are applied to thesurface.
 5. A method according to claim 1 in which the load is appliedby a tamper which is driven in about 10 to 20 percent of its equivalentdiameter at each impact.
 6. A method according to claim 1, whichincludes at least two successive cycles each comprising a dynamicpressure phase, followed by a rest phase.
 7. A method according to claim1, which includes an operation wherein draining materials are used toform the top part of the soil.